<?php
$data = array (
	array (
		'entity_id' => '880',
		'sku' => '100088',
		'name' => 'Tablet PC 512MB RAM 8GB Memory Wi-Fi Camera(Black)',
		'price' => '117.8300',
		'item' => array (
			array (
				'attribute_id' => '103',
				'frontend_label' => 'Meta Title',
				'value' => NULL,
			),
			array (
				'attribute_id' => '972',
				'frontend_label' => 'Features',
				'value' => '131,201,129',
			),
			array (
				'attribute_id' => '992',
				'frontend_label' => 'Battery capacity',
				'value' => '5500mAh',
			),
		),
	),
)

foreach($data as &$item){
	foreach($item['item'] as &$v){
		if($v['frontend_label']== 'Features'){
			$v['value'] = '123,123,123'; 
		} 
	}
}
